The Picacho Peak is one of several metamorphic core complexes of western North America located in Arizona, and represents the northwest end of the Catalina metamorphic core complex, which includes the Rincon, Tortolita, and Santa Catalina Mountains. The ∼100m thick Miocene detachment shear zone is divided into three “plates” separated by two low-angle detachment faults. By performing combined structural, microstructural, and 3D and 2D strain analysis of the well-exposed Oracle granite mylonite, we are provided with insight on the deformation and mechanical evolution of the continental crust during extension associated with the exhumation of metamorphic core complex.

Microstructural and Electron backscattered diffraction observations show transition from high stress microstructures in the lower plate, to cataclastic flow in the middle and upper plate. 3D and 2D strain analyses reveal that strain is mostly in the flattening field, suggesting a strong component of coaxial flow. The recrystallized grain size of quartz is relatively small (18.1 to 21.8μm), yielding high flow stress estimates ranging from 58 to 68 MPa. Altogether, these results suggest that the DSZ evolved close to its peak strength, close to the dislocation creep/exponential creep transition.